1. Field of the Invention
This invention relates to accelerometers, and in particular, to a silicon micromechanical accelerometer having integral bidirectional shock protection and viscous damping.
2. Description of the Prior Art
Many micromechanical devices are now well known. Such devices includes sensors of all types, for example, for sensing force, pressure, acceleration, chemical concentration, etc. These devices are termed "micromechanical" because of their small dimensions--on the order of a few millimeters square or smaller. The small size is achieved by employing photolithographic technology similar to that employed in the fabrication of integrated circuits. With this technology, the devices are as small as microelectronic circuits, and many such devices often are fabricated in a batch on a single wafer or other substrate, thereby spreading the cost of processing that wafer among many individual devices. The resulting low unit cost enormously increases the applications for such devices. In addition, by forming such devices on a semiconductor substrate such as a silicon wafer, associated control and/or sensing circuitry may be formed on the same substrate during the same processes, thereby further increasing density and reducing cost.
One type of micromechanical device--an accelerometer--has been fabricated by a number of individuals. A silicon accelerometer developed by Lynn M. Roylance is reported in Scientific American (April 1983) 248(4):44-55, in an article entitled, "Silicon Micromechanical Devices" by J. B. Angell, S. C. Terry and P. W. Barth, one of the inventors herein. That article describes the use of selective chemical etching to create an accelerometer having a mass of silicon suspended at the end of a thin silicon beam in which a resistor is formed. Subjected to acceleration, the mass is displaced, causing flexure of the beam, thereby changing the resistance of the resistor. The change of resistance provides an accurate measure of the acceleration. Other well known micromechanical accelerometers are described in the prior art statement accompanying this application.
Several market forces have pushed the development of low-cost batch-fabricated accelerometers over the past few years. Primary along these are automotive needs for crash sensors for air bag deployment and ride motion sensors for active suspension components. Additional markets include military components such as smart weapons and aviation instruments such as rate-of-climb indicators. Accelerometer development efforts have pushed for medium performance (less than required for inertial navigation or gravitometers), reproducible characteristics, and low cost. The primary obstacle to the development of such sensors has been the fragility of the fabricated devices: adequate sensitivity for low accelerations (0.5-1 G) has historically resulted in easy breakage of the sensor during and after fabrication. This breakage problem drives yield down and price up.
Unfortunately, such prior art acceleration sensors have suffered from a number of disadvantages which have raised their cost of manufacture, limited their accuracy, and precluded their use in many applications. For example, for greater sensitivity to small acceleration forces, the cantilever beam suspending the mass must be more flexible, thereby requiring stops to control the displacement of the mass when subjected to larger acceleration forces. The stops prevent the mass from being displaced to the extent that the cantilever beam cracks, rendering the structure nonfunctional. In the prior art such stops were provided by separate mechanical means such as plates or arms supported externally of the moving part of the acceleration sensor, and supporting frame, thereby requiring additional manufacturing, alignment and adjustment steps, each adding cost to the product while reducing yield. Even with the additional process requirements, the accuracy of such stops is not as great as desired. Furthermore, it was almost impossible to match the travel distance of the mass in each direction (positive or negative g's) because of the difficult adjustment and alignment presented by the external stops. Such external stops also trap dust particles between the mass and the stops during fabrication of the stops, again degrading yield. A typical prior art approach for the formation of stops is described by M. Nakamura, et al., "Novel Electrochemical Micro-Machining and Its Application for Semiconductor Acceleration Sensor IC," Digest of Technical Papers, 4th International Conference on Solid-State Sensors and Actuators, Institute of Electrical Engineers of Japan, pp. 112-115.
A further disadvantage of prior art micromachined acceleration sensors is the difficulty of providing adequate damping. Damping of the sensor is required for use of the sensor near its resonant frequency. In the prior art damping was provided by the introduction of oil or other viscous fluid into the sensor. This made control of the precise damping characteristic difficult. Furthermore, the damping provided by these techniques was often at least partially dependent on the stop characteristics, thereby precluding independent control of damping and travel distance.